52 patients with posterior ankle impingement due to an os trigonum were randomized to either endoscopic or open surgery for excision of the os trigonum. Groups were compared for the incidence of postoperative complications, time to return to training, and time to return to sport. Secondary outcomes also included patient-reported measures of the American Orthopaedic Foot and Ankle hindfoot score for function, and the Visual Analog Score-Foot and Ankle score for pain. Results demonstrated a significantly lower incidence of complications in the endoscopic group compared to the open group, as well as significantly shorter times to return to both training and preinjury sporting level. The endoscopic group also demonstrated significantly better scores compared to the open group on the AOFAS-hindfoot score at 6 months and 5 years, and on the VAS-FA score at 6 months.